type texenv_param =
  | GL_ADD
  | GL_ADD_SIGNED
  | GL_INTERPOLATE
  | GL_MODULATE
  | GL_DECAL
  | GL_BLEND
  | GL_REPLACE
  | GL_SUBTRACT
  | GL_COMBINE
  | GL_TEXTURE
  | GL_CONSTANT
  | GL_PRIMARY_COLOR
  | GL_PREVIOUS
  | GL_SRC_COLOR
  | GL_ONE_MINUS_SRC_COLOR
  | GL_SRC_ALPHA
  | GL_ONE_MINUS_SRC_ALPHA
  | GL_TRUE
  | GL_FALSE
